Add support for pages in the union/intersect template function

[digitalcraftsman]

Complex where-nestings could be simplified by filtering pages with union and intersect:

• union corresponds to the logical OR operator (x is an element of set A OR set B)
• intersect corresponds to the logical AND operator (x is an element of set A AND set B)

Example: find all pages where .Params.a equals "v1" AND .Params.b equals "v2"

{{ $v1 := where .Site.Pages "Params.a" "v1" }}
{{ $v2 := where .Site.Pages "Params.b" "v2" }}

{{ $filtered := $v1 | intersect $v2 }}

[bep]

Note that instead of adding support for one and one type, it should support any type that can be compared by == and !=.

And I think you switched AND and OR in the bullet points above.

[digitalcraftsman]

Note that instead of adding support for one and one type, it should support any type that can be compared by == and !=.

Makes sense

And I think you switched AND and OR in the bullet points above.

The typical copy-and-paste dilemma. It's fixed now.

[bep]

@moorereason @digitalcraftsman The intersect func should be fine now, right? It would be cool if we could also get the union up to speed? That would give us a good story on page queries.

[digitalcraftsman]

Great! We should also add some nice examples to the docs. We should link from the docs for the where template func to examples for union and intersection.

/cc @rdwatters

[bep]

@digitalcraftsman my post was riddled with question marks, which may be good to address before we start worrying about examples.

[moorereason]

The union enhancements are tracked in #3411. I hope to get to those in the v0.21 time frame (feel free to claim that issue if you want to do it!). They should look much like the intersect updates in #3328.

I still don't think we support Page comparisons. We are doing comparisons of basic types. We don't compare structs. I looked at comparing Pages a few days ago and ran into an issue with cyclical imports since tpl/collections would need to import hugolib.

[bep]

@moorereason re the cyclic, I had a Git stash laying around, pushed it in #3484 -- I probably will not complete this, so take what you want ...

You don't really need to test real pages, just make up some structs and slices of struct pointers.

[Anastasiia-XFR]

If there two range, for example:
{{ $arr1 := slice "AU" "DE" "GB" "IT" "FR" }}
{{ $arr2 := slice "RU" "DE" "FR" }}
How can I find different element, for this case "GB" and "IT"?

Oh, I find it
{{ $res := $arr1 | symdiff $arr2 }}
{{ $res }} or {{ $res = uniq $res | sort }}